qemu: prevent use of <bios useserial='yes'> on non-x86 arches
qemu: prevent use of <bios useserial='yes'> on non-x86 arches

The <bios useserial='yes'> config results in use of the '-device sga'
QEMU options. This in turn causes QEMU go load the sgabios.bin option
ROM, which contains x86 machine code. This cannot work on non-x86
arches, thus we should block the bad config.

Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

olafhering/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt